Atmospheric Particle Counting

ABSTRACT

Atmospheric particle detectors having a hybrid measurement cavity and light baffle are provided. In one aspect, an atmospheric particle detector includes: an optical measurement cavity; a light baffle attached to the optical measurement cavity, wherein the light baffle is configured to i) permit unobstructed airflow into the optical measurement cavity and ii) block ambient light from entering the optical measurement cavity; a photodetector on a first side of the optical measurement cavity; a retro reflector on a second side of the optical measurement cavity opposite the photodetector, and a light source configured to produce a light beam that passes through the optical measurement cavity without illuminating the photodetector. A method for particle detection using the atmospheric particle detector is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/238,412 filed on Aug. 16, 2016, the disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to atmospheric particle detectors, andmore particularly, to atmospheric particle detectors having a hybridmeasurement cavity and light baffle and techniques for use thereof.

BACKGROUND OF THE INVENTION

Atmospheric particle counting is an important component of overallpollution monitoring. The data on particle size and concentration areessential to air pollution source identification and forecasting.

Many cities and urban areas have evolved highly polluted atmospheres.Accordingly, it is desirable to have a large number of sensors operatingautonomously to collect long term, high resolution (concentrated) data.However, due to high levels of pollution, the sensor devices need to beresilient to particle build-up in order to afford long term service.

Accordingly, a particle counter that collects long term, high resolutiondata even in highly polluted atmospheres would be desirable.

SUMMARY OF THE INVENTION

The present invention provides atmospheric particle detectors having ahybrid measurement cavity and light baffle and techniques for usethereof. In one aspect of the invention, an atmospheric particledetector is provided. The atmospheric particle detector includes: anoptical measurement cavity; a light baffle attached to the opticalmeasurement cavity, wherein the light baffle is configured to i) permitunobstructed airflow into the optical measurement cavity and ii) blockambient light from entering the optical measurement cavity; aphotodetector on a first side of the optical measurement cavity; a retroreflector on a second side of the optical measurement cavity oppositethe photodetector; and a light source configured to produce a light beamthat passes through the optical measurement cavity without illuminatingthe photodetector.

In another aspect of the invention, a method for particle detection isprovided. The method includes: providing an atmospheric particledetector, comprising: an optical measurement cavity, a light baffleattached to the optical measurement cavity, a photodetector on a firstside of the optical measurement cavity, a retro reflector on a secondside of the optical measurement cavity opposite the photodetector, and alight source configured to produce a light beam that passes through theoptical measurement cavity without illuminating the photodetector;introducing an air sample into the optical measurement cavity throughthe light baffle, wherein the light baffle permits unobstructed airflowinto the optical measurement cavity and blocks ambient light fromentering the optical measurement cavity; passing the light beam throughthe air sample in the optical measurement cavity between thephotodetector and the retro reflector, wherein light from the light beamis scattered off of particles in the air sample to the photodetector orto the retro reflector which reflects the light back toward thephotodetector; and detecting the light scattered off of the particlesusing the photodetector.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary atmospheric particledetector according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary system incorporating theatmospheric particle detector of FIG. 1 according to an embodiment ofthe present invention;

FIG. 3 is a diagram illustrating an exemplary methodology for particledetection using the present atmospheric particle detector and simplediffusion of air samples into the detector according to an embodiment ofthe present invention;

FIG. 4 is a diagram illustrating an exemplary methodology for particledetection using the present atmospheric particle detector and activeingestion of a measured amount of air sample using an air pump accordingto an embodiment of the present invention;

FIG. 5 is a diagram illustrating an exemplary methodology for particledetection using the present atmospheric particle detector and continuousactive ingestion of air samples using an air pump according to anembodiment of the present invention;

FIG. 6 is a diagram illustrating a detected light pulse from laser lightscattered from particles according to an embodiment of the presentinvention; and

FIG. 7 is a diagram illustrating an exemplary apparatus that can beimplemented to serve as the processor in the system of FIG. 2 accordingto an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A light- or laser-based optical particle counter operates by providing aconstant sample airflow through an optical cavity where the airflowpasses through a light/laser beam. Light scatters off of the particlesin the airflow sample and this scattering is detected by aphotodetector. Over an extended period of time, however, withconventional optical particle counters particles build up on the wallsof the optical cavity causing a constant level of scattered light to becreated. This constant accumulation eventually builds up to the pointthat the instrument is inoperable. In high pollution areas, thisaccumulation can occur quite rapidly.

Provided herein are light- or laser-based particle counting deviceswherein a sample airflow cavity is constructed so as to allow outsideair to diffuse in through a light baffle or to be actively drawn inaccording to the air particle concentration and the desired measurementinterval. Advantageously, this light-baffled and optional active pumpdesign, allows for managing the amount of air ingested (i.e., drawninto) in the optical cavity to minimize the amount of particleaccumulation.

Minimization of particle accumulation can be accomplished by coatingexposed surfaces with hydrophobic materials (see below), designing theoptical cavity to eliminate fluid stagnation points with respect to theairflow, and most fundamentally reducing the amount of ingested air. Fora given cavity configuration and ambient particle concentration, it isreasonable to conclude that lifetime is proportional to the volume ofingested air.

Further, as will be described in detail below, one can thus adjust theamount of air ingested into the device based on the particle contentbeing observed to minimize the amount of exposure of the optical cavityto pollution. For instance, as the particle count increases due to highpollution, the air sample size can be decreased accordingly to preventover contamination of the device.

Also, given the goal of operating the device remotely and over extendedperiods of time, it is desirable to minimize the overall powerconsumption of the device. As will be described in detail below,embodiments are provided herein where low power consumption componentsare employed, such as a micro diaphragm pump that consumes, e.g., about40 milliamps of current at 3.3 volts.

Therefore, the present particle detection device is optimal for longlife in highly polluted environments in that: 1) the device is able tooperate by diffusion only (or optionally under active pumping; 2) thedevice can operate by drawing one cavity volume of air at a time toreduce the time to equilibrium with the outside air; and 3) when air isdrawn continuously, it can be for short precise durations.

FIG. 1 is a diagram illustrating an atmospheric particle detector 100according to an exemplary embodiment. Particle detector 100 includes ahybrid airflow intake/optical measurement cavity that is open to theexternal environment. Namely, as shown in FIG. 1, the hybrid cavity hasa light baffle 102 that is open to airflow 104 from the externalenvironment but which blocks ambient light from entering the opticalmeasurement cavity 106. The optical measurement cavity 106 is connectedto the light baffle 102 such that outside airflow 104 can diffuse or beactively pumped/drawn into the optical measurement cavity 106. Theoptical measurement cavity 106 is configured such that a light or laserbeam from a light source (see FIG. 2—described below) can be transmittedthrough the optical measurement cavity 106 whereby particles entrainedin the air sample within the optical measurement cavity 106 scatterlight 108 in detectable view of a photodetector 110 and/or to a retroreflector 112 (located on a side of the optical measurement cavity 106opposite the photodetector 110) which reflects the light back towardsthe photodetector 110.

According to an exemplary embodiment, the photodetector 110 includes aphotodiode, an avalanche diode, and/or a photomultiplier tube.Photodiodes convert light into current when photons are absorbed in thephotodiode. An avalanche diode is a semiconductor-based diode in whichavalanche multiplication of charge carriers occurs. See, for example,U.S. Pat. No. 3,921,192 issued to Goronkin et al., entitled “AvalancheDiode,” the contents of which are incorporated by reference as if fullyset forth herein. Photomultiplier tubes operate by amplifying electronsgenerated by a photocathode that are exposed to a photon flux.

The optical measurement cavity 106 is transparent to light. Namely, asshown in FIG. 1, the scattered light 108 within the optical measurementcavity 106 can pass through a first transparent window (labeled “window1”) toward the photodetector 110 and/or through a second transparentwindow (labeled “window 2”) toward the retro reflector 112. According toan exemplary embodiment, first transparent window 1 and secondtransparent window 2 are located on opposite sides of the opticalmeasurement cavity 106 in line of sight of one another. That way, lightis reflected off of the retro reflector 112 back towards thephotodetector 110 (thereby amplifying the signal 116). The firsttransparent window 1 and second transparent window 2 preferably form anairtight seal with the walls of the optical measurement cavity 106thereby limiting airflow into the optical measurement cavity 106 to thatthrough the light baffle 102. Suitable materials for forming the firsttransparent window 1 and the second transparent window 2 include, butare not limited to, glass, transparent plastics, etc.

According to an exemplary embodiment, inner surfaces of the opticalmeasurement cavity 106 are lined with a super hydrophobic coating tominimize the attachment and accumulation of particles on the wallsand/or other surfaces of the cavity. By way of example only, the superhydrophobic coating can include a monolayer (i.e., a layer that is oneatom thick) of a fluorinated silated compound. Examples include avariety of surface-modifying compounds offered by Gelest, Inc.,Morrisville, Pa. such as fluoroalkylsilanes: SIT8371.0((3,3,3-trifluoropropyl) trichlorosilane), SIT8174.0((tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane), and/orSIH5841.0 ((heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane)that can be applied to form a surface layer on the inner walls of thecavity.

As shown in FIG. 1, the scattered light 108 within the opticalmeasurement cavity 106 is further directed to the photodetector 110 by aretro reflector 112 thereby increasing the size of the scattered lightsignal. The term “retro reflector” as used herein generally refers toany means for reflecting light back towards its source. According to anexemplary embodiment, the retro reflector 112 includes a reflectivetape, reflective paint and/or a reflective composite material. Examplesof these materials include tapes and paints (often used in roadwayreflectors) that incorporate titanated (some fraction of titanium oxide)glass spheres. By including titanium oxide in the glass formulation, itis possible to achieve an optical index close to 2.0. A sphere with thisoptical index will retro reflect from its rear surface. The reflectionat the back interface of the sphere is often enhanced by using anadhesive that favorably affects the index mismatch with respect to thesphere. Alternative embodiments of retro reflectors include tapes with afirst surface of clear plastic embossed with prismatic tessellationsthat retro reflect light similar to corner reflectors.

The light baffle 102 is configured to permit (unobstructed) airflow intothe optical measurement cavity 106 while at the same time block outsidelight from entering the optical measurement cavity 106. For instance, inthe example shown illustrated in FIG. 1, the light baffle 102 is asemicircular-shaped conduit which freely permits airflow into or out ofthe optical measurement cavity 106. However, as shown in FIG. 1, lightcannot enter the optical measurement cavity 106 through the light baffle102. Preferably, the interior surfaces of the light baffle 102 arenon-reflective, e.g., lined with a non-reflective coating, so as toprevent reflection of incident light into the cavity.

A port 114 is connected to the hybrid cavity to allow an external airpump (see, e.g., FIG. 2—described below) to serve as an optional meansfor actively drawing air into the cavity. Suitable air pumps include,but are not limited to, micro diaphragm pumps, micro peristaltic pumps,and/or micro positive displacement pumps. As provided above, for longterm service it is desirable for the device to consume minimal amountsof power during operation. For instance, a micro diaphragm pump consumesless than or equal to about 40 mA of current at about 3.3 volts.Suitable micro diaphragm pumps are available commercially, e.g., fromParker-Hannifin, Cleveland, Ohio.

Light incident to the photodetector 110 causes an electrical signal 116to be produced. As shown in FIG. 1, this signal is further amplified andconditioned by a circuit 118 to produce a detectable pulse 120 that isultimately digitized, measured and counted. The data so produced is usedto estimate the particle size and concentration in the immediateenvironment.

An exemplary system 200 is shown illustrated in FIG. 2 that incorporatesthe atmospheric particle detector 100 of FIG. 1. As shown in FIG. 2, theairflow port 114 can be connected to (at least one) air pump 202. Asprovided above, a suitable air pump includes a micro diaphragm pump, amicro peristaltic pump, and/or a micro positive displacement pump.

Preferably, a filter 204 is present between the airflow port 114 and theair pump 202 to capture particles exiting the atmospheric particledetector 100 via the airflow port 114. Namely, as will be described indetail below, several different modes of operation may be employed. Inone mode, simple diffusion of the airflow sample into the opticalmeasurement cavity 106 is employed. See, e.g., FIG. 3—described below.In that case, no action of the air pump 202 is needed.

In another mode, the air pump 202 is used to actively draw the airsample into the optical measurement cavity 106. Specifically, the airpump 202 can be used to draw air out of the optical measurement cavity106 via the airflow port 114 which serves to draw outside air into theoptical measurement cavity 106 via the light baffle 102. In oneexemplary embodiment, the air pump 202 is used to draw a measured amount(which can be regulated based on the length of time the air pump 202 isturned on) of an air sample into the optical measurement cavity106—after which the pump is turned off. See, e.g., FIG. 4—describedbelow. Alternatively, in another exemplary embodiment, the air pump 202is operated continuously generating a constant airflow sample throughthe optical measurement cavity 106. See, e.g., FIG. 5—described below.

FIG. 2 also illustrates how a light source 206 can be positioned tointroduce a light or laser beam 208 into the optical measurement cavity106. Namely, the light source 206 can be located within the opticalmeasurement cavity 106 or outside of the optical measurement cavity 106,as long as the light or laser beam 208 passes between the firsttransparent window (labeled “window 1”) and the second transparentwindow (labeled “window 2”) in line of sight of the photodetector 110.Suitable light sources 208 include, but are not limited to, a diodelaser, a gas laser, a light emitting diode (LED) and/or an incandescentsource. It is notable that, as shown in FIG. 2, the light or laser beam208 produced by the light source 206 passes through the opticalmeasurement cavity 106 in a manner so as to illuminate the opticalmeasurement cavity 106 and thus the particles within the opticalmeasurement cavity 106, but not illuminate the photodetector 110. Asdescribed above, it is the light from beam 208 that is scattered by theparticles in the air sample that illuminates the photodetector 110(and/or the scattered light reflected back by the retro reflector). Todo so, as shown in FIG. 2, the beam 208 can pass perpendicular to theline of sight between the photodetector 110 and the retro reflector 112.In the exemplary embodiment shown in FIG. 2, a processor 210 (such asapparatus 700 with data processing capabilities—see for example FIG. 7,described below) is used to digitize the conditioned signal from circuit118.

As provided above, the present atmospheric particle detector system canbe operated in different modes. For instance, in one (first) modepassive air diffusion and Brownian motion through the light baffle 102is used to introduce air samples into the optical measurement cavity106. See, e.g., methodology 300 of FIG. 3. Namely, the light baffle isopen to outside air and in step 302 air simply diffuses into the opticalmeasurement cavity 106.

In step 304, the light or laser beam 208 is passed through the airsample in the optical measurement cavity 106 and light from the beam 208is scattered off of the particles in the air sample. In step 306, thescattered light is detected by the photodetector 110 which produces anelectrical signal. As described above, the retro reflector 112 reflectslight scattered in the optical measurement cavity 106 back toward thephotodetector 110.

In step 308, the electrical signal from the photodetector 110 isamplified and conditioned using the circuit 118. In step 310, theconditioned signal is digitized and processed by processor 210.

In another exemplary (second) mode of operation described by way ofreference to methodology 400 of FIG. 4, air is ingested into the opticalmeasurement cavity 106 (through the light baffle 102) using air pump 202and, after an amount of air sufficient to fill the cavity is ingestedinto the optical measurement cavity 106, the air pump 202 is stopped.See step 402. The particle detection then proceeds in the same manner asdescribed above.

Namely, in step 404, the light or laser beam 208 is passed through theair sample in the optical measurement cavity 106 and light from the beam208 is scattered off of the particles in the air sample. In step 406,the scattered light is detected by the photodetector 110 which producesan electrical signal. As described above, the retro reflector 112reflects light scattered in the optical measurement cavity 106 backtoward the photodetector 110.

In step 408, the electrical signal from the photodetector 110 isamplified and conditioned using the circuit 118. In step 410, theconditioned signal is digitized and processed by processor 210.

In yet another exemplary (third) mode of operation described by way ofreference to methodology 500 of FIG. 5, air is ingested into the opticalmeasurement cavity 106 (through the light baffle 102) using air pump 202and is processed continuously. See step 502. By comparison withmethodologies 300 and 400 described above which use diffusion andBrownian motion, in methodology 500 particles move through the opticalmeasurement cavity 106 at a velocity proportional to the pump speed andthe size of the optical measurement cavity 106. The particle detectionis performed in the same manner as described above. It is notable thatthe air sampling (step 502) and the particle detection (steps 504-510)are performed simultaneously and continuously in this example.

Namely, in step 504, the light or laser beam 208 is passed through theair sample in the optical measurement cavity 106 and light from the beam208 is scattered off of the particles in the air sample. In step 506,the scattered light is detected by the photodetector 110 which producesan electrical signal. As described above, the retro reflector 112reflects light scattered in the optical measurement cavity 106 backtoward the photodetector 110.

In step 508, the electrical signal from the photodetector 110 isamplified and conditioned using the circuit 118. In step 510, theconditioned signal is digitized and processed by processor 210.

The detected particles in the third mode (i.e., methodology 500) have anarrower pulse width than those detected in the first and second modes(i.e., methodologies 300 and 400). Each mode of operation can bebeneficial for a given set of conditions. For instance, the third mode(i.e., methodology 500) is preferable in low particle concentrationenvironments. The first and second modes (i.e., methodologies 300 and400) are preferable in medium to high particle concentrationenvironments.

FIG. 6 is a diagram illustrating an electrical signal generated by thephotodetector 110. As shown in FIG. 6, the signal amplitude is plottedas a function of time. As provided above, suitable photodetectorsinclude, but are not limited to, photodiodes, avalanche diodes, and/orphotomultiplier tubes.

Turning now to FIG. 7, a block diagram is shown of an apparatus 700 thatcan be implemented to serve as processor 210 (see FIG. 2, describedabove). Apparatus 700 includes a computer system 710 and removable media750. Computer system 710 includes a processor device 720, a networkinterface 725, a memory 730, a media interface 735 and an optionaldisplay 740. Network interface 725 allows computer system 710 to connectto a network, while media interface 735 allows computer system 710 tointeract with media, such as a hard drive or removable media 750.

Processor device 720 can be configured to implement the methods, steps,and functions disclosed herein. The memory 730 could be distributed orlocal and the processor device 720 could be distributed or singular. Thememory 730 could be implemented as an electrical, magnetic or opticalmemory, or any combination of these or other types of storage devices.Moreover, the term “memory” should be construed broadly enough toencompass any information able to be read from, or written to, anaddress in the addressable space accessed by processor device 720. Withthis definition, information on a network, accessible through networkinterface 725, is still within memory 730 because the processor device720 can retrieve the information from the network. It should be notedthat each distributed processor that makes up processor device 720generally contains its own addressable memory space. It should also benoted that some or all of computer system 710 can be incorporated intoan application-specific or general-use integrated circuit.

Optional display 740 is any type of display suitable for interactingwith a human user of apparatus 700. Generally, display 740 is a computermonitor or other similar display

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. An atmospheric particle detector, comprising: anoptical measurement cavity; a light baffle attached to the opticalmeasurement cavity, wherein the light baffle is configured to i) permitunobstructed airflow into the optical measurement cavity and ii) blockambient light from entering the optical measurement cavity; aphotodetector on a first side of the optical measurement cavity; a retroreflector on a second side of the optical measurement cavity oppositethe photodetector; and a light source configured to produce a light beamthat passes through the optical measurement cavity without illuminatingthe photodetector.
 2. The atmospheric particle detector of claim 1,wherein interior surfaces of the light baffle are non-reflective.
 3. Theatmospheric particle detector of claim 1, wherein the photodetector isselected from the group consisting of: a photodiode, an avalanche diode,a photomultiplier tube, and combinations thereof.
 4. The atmosphericparticle detector of claim 1, wherein the light source is selected fromthe group consisting of: a diode laser, a gas laser, a light emittingdiode (LED), an incandescent source, and combinations thereof.
 5. Theatmospheric particle detector of claim 1, further comprising: a firsttransparent window in the first side of the optical measurement cavitythrough which light can pass between the optical measurement cavity andthe photodetector; and a second transparent window in the second side ofthe optical measurement cavity through which light can pass between theoptical measurement cavity and the retro reflector.
 6. The atmosphericparticle detector of claim 1, wherein an inner surface of the opticalmeasurement cavity is lined with a super hydrophobic coating.
 7. Theatmospheric particle detector of claim 6, wherein the super hydrophobiccoating comprises a fluoroalkylsilane selected from the group consistingof: 3,3,3-trifluoropropyl trichlorosilane,(tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane,(heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane, andcombinations thereof.
 8. The atmospheric particle detector of claim 1,wherein the retro reflector is selected from the group consisting of:reflective tape, reflective paint, a reflective composite material, andcombinations thereof.
 9. The atmospheric particle detector of claim 1,further comprising: an air pump connected to the optical measurementcavity via an airflow port.
 10. The atmospheric particle detector ofclaim 9, wherein the air pump is selected from the group consisting of:a micro diaphragm pump, a micro peristaltic pump, and a micro positivedisplacement pump.
 11. The atmospheric particle detector of claim 9,further comprising: a filter between the optical measurement cavity andthe air pump.
 12. The atmospheric particle detector of claim 1, furthercomprising: a signal conditioner configured to condition electricalsignals from the photodetector; and a processor configured to digitizethe electrical signals from the signal conditioner.
 13. A method forparticle detection, comprising: providing an atmospheric particledetector, comprising: an optical measurement cavity, a light baffleattached to the optical measurement cavity, a photodetector on a firstside of the optical measurement cavity, a retro reflector on a secondside of the optical measurement cavity opposite the photodetector, and alight source configured to produce a light beam that passes through theoptical measurement cavity without illuminating the photodetector;introducing an air sample into the optical measurement cavity throughthe light baffle, wherein the light baffle permits unobstructed airflowinto the optical measurement cavity and blocks ambient light fromentering the optical measurement cavity; passing the light beam throughthe air sample in the optical measurement cavity between thephotodetector and the retro reflector, wherein light from the light beamis scattered off of particles in the air sample to the photodetector orto the retro reflector which reflects the light back toward thephotodetector; and detecting the light scattered off of the particlesusing the photodetector.
 14. The method of claim 13, wherein thephotodetector, upon detecting the light scattered off of the particles,produces an electrical signal, the method further comprising:conditioning the electrical signal; and digitizing the electricalsignal.
 15. The method of claim 13, wherein the air sample is permittedto diffuse into the optical measurement cavity through the light baffle.16. The method of claim 13, wherein the atmospheric particle detectorfurther comprises an air pump connected to the optical measurementcavity via an airflow port, the method further comprising: drawing theair sample into the optical measurement cavity using the air pump; andturning off the air pump once the air sample fills the opticalmeasurement cavity.
 17. The method of claim 13, wherein the atmosphericparticle detector further comprises an air pump connected to the opticalmeasurement cavity via an airflow port, the method further comprising:continuously drawing air samples into the optical measurement cavityusing the air pump.
 18. The method of claim 13, wherein thephotodetector is selected from the group consisting of: a photodiode, anavalanche diode, a photomultiplier tube, and combinations thereof. 19.The method of claim 13, wherein an inner surface of the opticalmeasurement cavity is lined with a super hydrophobic coating.
 20. Themethod of claim 19, wherein the super hydrophobic coating comprises afluoroalkylsilane selected from the group consisting of:3,3,3-trifluoropropyl trichlorosilane,(tridecafluoro-1,1,2,2-tetrahydrooctyl) trichlorosilane,(heptadecafluoro-1,1,2,2-tetrahydrodecyl) trichlorosilane, andcombinations thereof.